China's air pollution, according to The Lancet, leads to 1.1 million premature deaths from respiratory and autoimmune diseases per year. A new study of geologists from the Chinese University of Geology in Wuhan shows, however, that air pollution can be not only the cause of disease, but also a natural disaster. It turned out that acid rain, falling due to the high content of coal combustion products in the atmosphere, slowly softens some types of rocks, which can cause landslides and mudslides.

Landslides are usually caused by the fact that an external force - such as strong precipitation or earthquakes - destabilizes rock layers that form a mountain or a hill. Earthquakes inform the breed of mechanical energy necessary to overcome the frictional forces that hold the mountain in place, and water serves as a lubricant and reduces frictional forces themselves.

The 2009 landslide in Jiweishan landslide, which killed 74 people, happened for no apparent reason: there were no heavy rains, no earthquakes. The authors of the article, which is preparing for publication in the journal Earth and Planetary Science Letters, found the reason - it was acid rain, slowly dissolved one of the layers of the mountain. The reason for the acidity of the rains, in turn, was the burning of coal (China is the world's main importer of coal, its burning is the main way of generating electricity in the country). Among the products of coal combustion are oxides of sulfur and nitrogen, which give strong acids in contact with water in the clouds.

Acid rain is considered, if its pH is equal to 5 or less, the normal pH of the rainwater is 5.6. From 1986 to 2014, the average for the region where the landslide occurred was between 4.3 and 5.

To make sure that the cause of the landslide was precisely the acid rain, the geologists turned to the structure of the mountain with which the disaster began. Part of it was formed by black clay containing slippery minerals (for example, talcum), and calcite clinging clay. Water with a low pH could dissolve the calcite and make the clay very slippery. The assumption was confirmed by a laboratory experiment, but not only this chemical reaction caused a landslide; acid water created in the rock a surplus of oxygen, nitrogen, phosphorus and sulfur.

In this environment, microorganisms living in clay were more active than usual;

analysis of DNA extracted from clay samples showed the presence of hundreds of microorganism species, including those that are capable of decomposing some components of clay. Getting enough nutrients with sour water, such microorganisms could literally exacerbate the mountain, says one of the authors of the study, Mauri McSavini, a geomorphologic engineer from the Royal New Zealand Research Institute. Now the authors of the work plan to find more evidence of their hypothesis by studying the microbiota and acidity of rainwater in other places where for no apparent reason there were large landslides.